The present invention relates to a method for encapsulating a semiconductor device, such as transistors and LSIs, with a resin composition and a novel resin composition therefor.
As is well known, various kinds of semiconductor devices used in the technology of electronics are packaged with a ceramic material or encapsulated with a resin composition in order to be protected from the adverse influences of environment such as moisture, heat, dusts, stains and the like as well as from mechanical damages. The method of resin encapsulation is preferred to the ceramic packaging owing to the lower costs and higher productivity of the method and various kinds of resin compositions have been proposed for such a purpose.
The resin composition for encapsulation of a semiconductor device is usually composed of a synthetic resin, which may be thermosetting or thermoplastic, and an inorganic filler, i. e. an inorganic finely divided partriculate material, dispersed in the resin as a matrix. The proportion of the inorganic filler to the resin matrix should be as large as possible in order to impart the resin composition with small coefficient of expansion, high thermal conductivity, low moisture permeability and excellent mechanical properties as well as from the standpoint of decreasing the costs of the resin composition.
Various kinds of inorganic fillers are currently used in the encapsulating resin compositions but the most preferred among them are the silica fillers in order to fully obtain the above mentioned advantages by the incorporation of an inorganic filler and most of high-performance encapsulating resin compositions are prepared by loading with a silica filler. Silica fillers may be classified into crystalline-type and amorphous-type ones having their own respective advantages and disadvantages suitable or detrimental for a particular resin composition to be used in the intended application.
Along with the remarkably rapid progress of the electronics trechnology in recent years leading to a more and more delicate performance of electronic instruments with, for example, higher and higher degree of integration in LSIs and VLSIs, there have come to the attention of experts certain undesirable phenomena of wrong operation of the instruments called soft errors, the reason for which cannot be other than the failure in the operation of the semiconductor devices built in the instrument. Extensive investigations for the mechanism of such a drawback have revealed that the failure in the operation of the semiconductor devices is caused by the influence of the alpha-particles emitted from a trace amount of the radioactive elements, e.g. uranium and thorium, contained in the silica material as a filler in the resin composition with which the semiconductor devices are encapsulated. This problem is so serious that, for example, the design of the 64K- to 256K-bit dynamic RAM and the like belonging to the earlier development must be influenced thereby. Accordingly, it has been eagerly desired to develop a method for preventing such an undesirable phenomenon due to the influences of radioactivity.
A method has been proposed for such a purpose in which an undercoating is provided on the surface of the semiconductor device before resin encapsulation with an unfilled resin composition such as polyimide resins, silicone resins and the like in such a thickness as not to permit penetration of the alpha-particles. Such a method is, however, not quite acceptable with unreliableness due to the difficulties in providing the undercoating layer having a uniform thickness and over a definite surface area and due to the troubles sometimes caused by the excessive strain or stress as a result of the difference in the thermal expansion coefficients between the surface of the semiconductor device and the undercoating resin and between the undercoating resin and the encapsulating resin composition thereover. In addition, such a method is necessarily disadvantageous from the standpoints of productivity and costs due to the increase in the number of steps in the process of resin encapsulation by the addition of the troublesome step of undercoating.
The above problem due to the radioactivity of the uranium and thorium in the silica filler has been construed unavoidable in so far as the silica filler is prepared from natural silica sources such as ground quartz since quartz and other silica minerals contain such radioactive impurities more or less.
Accordingly, there has been proposed the use of a man-made silica filler including the so-called fumed silica fillers prepared by the oxidative pyrolysis of a volatilizable silicon compound, e.g. silicon tetrachloride, in an oxyhydrogen flame and the precipitated silica fillers prepared by the neutralization of a hydrated sodium silicate with an acid to precipitate the silica followed by drying. Unfortunately, no promising results have been obtained thus far with such man-made silica fillers. The reasons therefor are presumably that such a silica filler is so finely divided that the particle diameter thereof is around 10 nm or so with a specific surface area of 50 m.sup.2 /g or larger to give a limitation to the maximum loading amount of the filler into the resin matrix and that the surface of the filler particles have a large amount of silanolic hydroxy groups bonded thereto resulting in an excessively high consistency of the resin composition filled therewith to cause difficulties in uniformly dispersing the filler into the matrix resin in a sufficiently large amount as desired. In addition, even when the silicon compound as the starting material of the precipitated silica filler such as the sodium silicate can be highly purified to be free from the radioactive impurities, the resultant silica filler is not free from the alkaline impurities originating in the starting sodium silicate so that the resin is subjected to the detrimental effects by the alkaline impurities. Therefore, it is a generally accepted conclusion that the man-made silica fillers are far from suitable as a filler in the resin composition for the encapsulation of semiconductor devices.
Thus, there has been known no satisfactory method for encapsulating a semiconductor device with a resin composition without suffering from the adverse influences of the radioactive impurities in the silica filler incorporated into the encapsulating resin composition for semiconductor devices.